Re: [RTnet-users] Re: Problem Round-Trip-Time

[Jan K. <ki...@rt...>]

Vareka, Bill wrote:
> Assume you have a node (master) that needs to send very high speed packets
> with minimal jitter to 10 slaves.  So we define 10 slave nodes and
> interleave a master slot between each slave slot.  If each slot is allocated
> 150 us, then the cycle time will be on the order of 3150 us (10 master
> slots, 10 different slave slots plus the sync slot).  Now, will an
> asynchronous send_rt() on the master side go out in the next available
> master slot or will it have to wait until the next cycle?  In other words,
> am I guaranteed that a message sent by my application on the master side
> will go out no later than 300 us from the send_rt() call (assuming nothing
> of higher priority is queued) or will it take until the next cycle which
> could be as long as ~3.15 ms?
> 

In your setup, the message can actually be delayed until the next cycle. 
The point is that TDMA maintains separate queues for all time slots. 
This helps to simplify the scheduling (see also the TDMA2 slides on the 
RTnet documentation page). But your application can explicitly select 
the output slot on message transmission. Of course, you will then always 
have to know which slot comes next.

If you do not know this, you can let the master issue synchronisation 
messages 10 times faster. You would have to create a TDMA setup like this:

master:
ip 10.0.0.1
cycle 400       (to leave the station some time handling the messages)
slot 0 0 1/1 [size] (default slot, optionally with your preferred MTU)

slave:
ip 10.0.0.2
slot 0 200 1/10 [size] (default slot, transmit every 10th cycle)

slave:
ip 10.0.0.3
slot 0 200 2/10 [size]

slave:
ip 10.0.0.4
slot 0 200 3/10 [size]
...

But your scenario made me think about a potential extension of TDMA2: 
adding the option to share a common queue between /equally/ sized slots 
on one station. This way you would be able to use the original cycle 
period again. Should be feasible - let's see when time allows to work on 
it...

Jan